Advances in computer technology and in computer-aided manufacturing methods have made possible the design of automated control systems for machine tools that permit the machine tool to meet flexible manufacturing requirements and to be substantially unmanned during operation. The increased cost of labor and energy, the growing shortage of skilled machine tool operators, and the needed increased productivity per machine tool have created a high demand for such automated control systems. To be reliable and productive and yet to be substantially unmanned, these automated control systems monitor various parameters of the machine tool operation to perform adaptive control of the machine tool and to provide diagnostic analysis of the machine tool operation. A typical automated control system can be seen in U.S. Pat. No. 4,078,195, issued Mar. 7, 1978, Adaptive Control System for Numerically Controlled Machine Tools, Mathias et al.
For machine tools having a rotating tool, the parameters usually monitored are the transverse force on the tool (e.g., that force that is transverse to the longitudinal axis about which the tool rotates) which is usually measured in orthogonal x and y directions, the axial force on the tool (e.g., that parallel to the tool axis) which is usually measured in a z direction orthogonal to the x and y directions, and the torque on the tool. These parameters are used for various purposes, such as machine overload protection, tool breakage and vibration detection, tool life and wear monitoring, machining time monitoring, detection of the engagement of the tool with the workpiece, detection of a defective, missing or incorrect tool, and achievement of a maximum rate for the machining operation.
Various types of apparatus and methods have been used in the past to measure transverse force, axial force, and torque. Considering now machine tools having a rotating tool, the tool is mounted in and rotatable with a tool holder that in turn is mounted in and rotatable with a spindle that is supported by bearings in a carriage of the machine and that is rotated by a spindle-drive motor. The workpiece is mounted on a table that is moved in an x direction by an x-drive motor, and the carriage is moved in y and z directions relative to the workpiece by respective y-drive and z-drive motors.
The most common technique used to measure the transverse force on the tool is by measuring reactive displacements of the spindle in the x and y directions. An example of a sensor using this technique can be seen in U.S. Pat. No. 3,602,090, issued Aug. 31, 1971, Milling Machine Control System and Milling Force Sensor Therefor, Whetham, in which a plurality of magnetic transducers are mounted on the carriage in proximity to and about the periphery of the spindle. By appropriately positioning the magnetic transducers and by connecting the magnetic transducers in appropriate bridge arrangements, electrical signals may be obtained that are proportional to x and y displacements of the spindle and that are therefore related to the x and y forces exerted on the tool. The accuracy of this type sensor in converting measured displacements into measured forces is limited, however, by a number of factors such as the quality of the spindle bearing, the consistency and symmetry of the spindle stiffness, the amount of runout or eccentricity that the spindle exhibits during rotation, and temperature. The sensitivity of this type of sensor also changes with variations in the distance from the point at which the transverse load is applied (or, the "plane of loading") to the front of the spindle, so that special procedures must be used in calibrating the sensor and in programming the associated machine tool to accommodate the different planes of loading that are encountered during a machining operation.
Another technique for the measurement of transverse forces is that seen in U.S. Pat. No. 3,728,595, issued Apr. 17, 1973, Feed Rate Control System for Milling Machines, Adams, in which a plurality of strain gauges are mounted either on an adapter between the tool holder and the spindle or directly on the spindle itself. By appropriately mounting the strain gauges and by connecting the strain gauges in appropriate bridge arrangements, electrical signals may be obtained that are proportional to the strain in the tool holder in the x and y directions and that are therefore related to the x and y forces exerted on the tool. To date, practical implementation of this technique has not been achieved because of its cost and because of the hostile environment in which the reliability of the strain gauges must be ensured.
Another technique for the measurement of transverse forces is to measure the torque produced by the x-drive and y-drive motors. In order to measure torque, the power consumed by the motor is measured (by a watts transducer), as is the motor speed, and the measured power is then divided by the measured speed in order to obtain measured torque. While this technique provides adequate force measurements in the case of relatively large and steady-state torque values, the sensitivity of this technique is limited (e.g., torques less than three percent of the maximum torque available usually cannot be resolved) and the time or frequency of this technique is also limited by a number of factors such as friction, acceleration, and the limited frequency response of the watts transducer (e.g., about five Hz).
Still another technique for the measurement of transverse forces is to mount a precision dynamometer between the workpiece and the table which moves the workpiece. The dynamometer includes a plate which has formed therein a number of weakened sections or "beams" that have strain gauges mounted thereon. Although this technique can provide accurate and precise transverse force measurements with good time or frequency response, it is not practical for machining operations because the dynamometer is costly and has a very limited range of workpiece sizes and weights that can be used.
Techniques for the measurement of axial force on a rotating tool are generally similar to those previously described. For example, axial force can be measured by a displacement sensor having magnetic transducers disposed in proximity to the spindle, by strain gauges mounted on the spindle, by measuring the torque of the z-drive motor, or by a precision dynamometer interposed between the workpiece and the table. These techniques for the measurement of axial force are subject to the disadvantages previously noted of similar techniques for the measurement of transverse force. Measurement of torque on a rotating tool is commonly made by measuring the torque of the spindle motor or by strain gauges mounted on the spindle, tool holder, or an adapter therebetween. These techniques of torque measurement are also subject to the disadvantages previously noted.
Considering now machine tools having a stationary tool (such as a lathe), the tool is mounted in a tool holder that in turn is mounted on a tool post, and the tool post is moved in three orthogonal directions relative to the workpiece by associated drive motors. The workpiece is mounted on and rotatable with a spindle which in turn is rotated by an associated drive motor. The parameters usually monitored during a machining operation are the forces along the three orthogonal directions in which the tool post is moved. The most common technique for the measurement of these forces is by the use of a precision dynamometer similar to those previously described that is mounted between the tool holder and the tool post, and another technique that is used is to mount strain gauges directly on either the tool holder or the tool post. These techniques are subject to the disadvantages of high cost, low reliability, and limitation on the range of tool or tool holder sizes and weights similar to those previously discussed.
As can be appreciated from the foregoing discussion, many of the various techniques that have been described can be used to directly or indirectly measure lateral and angular displacements of a member of the machine tool (for example, the lateral displacement of a spindle upon application of a transverse or axial force to the spindle or the angular displacement of the spindle in response to torque exerted on the spindle). The displacement measurements made by use of such techniques, however, are also subject to all of the disadvantages previously noted in conjunction with transverse and axial force measurements and torque measurements.
Very accurate and precise measurements of the lateral and angular displacements of a member of a machine tool can be made by the use of laser interferometers, but such a technique is extremely costly and is suitable only for laboratory and quality testing purposes.